Not too big, handy. I don't usually shoot zooms, but this is a good lens for walkabout.
I would rate it about 7.5 or 8. Very well built. DxO gave it decent marks, not fantastic, but decent.

Cons:

distortion at the wide end. expensive

somewhere there is a review that compares this lens to the Nikon 24-70 f2.8 and gives it equal ratings. I could not find it for this post. Distortion in RAW shots is obvious, I don't shoot jpeg, so I can't address that.
I only shoot raw, and will probably use this lens for less than serious work, scouting for scenes where I would return with more gear and
spend hours getting the right shot. For that, it is perfect. Pretty expensive, as usual for Zeiss.

Excellent image quality, colors are quite amazing with a circ. polarizer. I was trying this lens out against the Zeiss 18mm and the Rokinon 14mm for Sony on my A7 and my big Nikon. No contest. I especially like the 'look' one gets with the 15mm. Some think a polarizer not needed on such a lens, but I was impressed with what I got with it.
Also, the 18mm hot spots (slightly) on my A7r IR camera, the 15mm does not.
Lens cap is excellent.